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Bore capacity & expansion ratio.

The terms "bore capacity" and "expansion ratio" are among the most confusing in shooting lingo, and difficult to explain in simple words. But they are important to all handloaders, especially when designing wildcat cartridges, selecting propellant powders, or specifying a new rifle, new barrel, or rechambering job.

Expansion ratio is the more comprehensible of the two. It is simply that number of times powder gases can increase in volume, by a factor equal to the original volume of the powder, from the instant of powder ignition to the instant the bullet clears the muzzle and "uncorks" the sealed propulsion system. Expansion ratio is occasionally used incorrectly as a synonym for "efficiency", in that high expansion-ratio combinations are said to be highly efficient, and low ones less so. That may be true in one sense, but low expansion-ratio systems are almost always high-velocity systems.

The "inefficient" smallbore magnums such as the .264 Win., 7mm Rem., and .300 Weatherby are all low expansion-ratio rifles, with ERs around 5 or 6. The smaller cartridges with more modest velocity potentials--.22 Hornet, .256 Win. Magnum, .222 Rem., etc.--rate much higher on the ER scale: from 12 to 15. They are more efficient in the sense that they extract more useful velocity and energy from each grain of powder, but they cannot deliver the sheer power of the low-ER cartridge.

Expansion raio, obviously, varies with bore diameter, barrel length, and the relationship of neck size to powder capacity in the cartridge. If other things remain the same, a straight case has a higher expansion ratio than a bottlenecked one (which is really another way of saying that the bore will have more volume per unit of powder capacity in the case). Also, if all else is equal, a longer barrel gives a higher expansion ratio than a shorter one, simply because it has more internal volume and, thus, the gases evolving from the burning powder can multiply their volume more times before the bullet departs.

A firm grasp of the expansion-ratio concept makes a lot of things ballistically easier to understand--like, for example, why a .300 Weatherby must burn six times as much powder of identical energy-latent content to move a bullet of the same weight only twice as fast as an M-1 .30 Carbine can, even from a barrel only about two-thirds as long! This example involves 110-grain bullets, Du Pont IMR powders 4350 and 4227, respectively, (which do have the same energy content although they have different burning rates), and barrel lengths of 26 inches for the magnum versus 18 inches for the carbine.

Powder charges and approximate muzzle velocities are 90 grains and 4,000 feet per second (fps) for the Weatherby and 15 grains and 2,000 fps for the little M-1. In general, the smaller the ratio of powder charge weight to bullet weight, the higher the velocity potential of a hypothetical cartridge. It will, however, be a more expensive cartridge to load, less efficient in the customary usage of that term, with shorter case and barrel life at sustained maximum load levels, less flexible as to bullet weights in reduced loadings, and it will require a relatively longer barrel for satisfactory performance. Altogether, that's quite a bit of information to be gained about a cartridge which does not even necessarily exist from a simple bit of long division.

"Bore capacity" is really a sort of extension of the expansion-ratio concept, but it has been sadly kicked around in the gun press for so many years that the best that can be said is that shooters are confused about it. Cartridges have been accused of being "over-bore capacity", or simply "overbore", as though the quantity being described were inherent in the cartridge itself, which it isn't. It also involves barrel length, bullet sectional density, and the burning rate of the powder under discussion. With a barrel long enough, or a powder slow enough, no cartridge would be "overbore", and cartridges like the old wildcat form of the .25-06, which may have been badly overbore back in the '50s when the slowest propellant on the market was IMR 3031, are not necessarily so today, in the presence of Norma MRP, Winchester 785 Ball, or IMR or Hodgdon 4831. "Bore capacity", then, is not an obsolute, but a relative, term.

To understand it, even in relative terms, we must take a look at a couple of ballistic phenomena. In any cartridge, there is known to be a point of diminishing returns in charge weights, a point beyond which the addition of more powder contributes more and more pressure but less and less velocity per grain-weight of powder consumed. If the velocity and pressure curves are plotted on a graph, they will begin to diverge radically at this point, sometimes called the "delta point". The approach to this point in a series of progressively heavier and heavier charges can be observed on a chronograph, when each heavier load produces a smaller increment of velocity, until a point is reached at which another half-grain of propellant delivers little or no more muzzle velocity but a possibly-disastrous increase in breech pressure.

A very small case with a relatively heavy bullet may not have enough room in it for enough of any of the powders considered normal for it to reach this point, and such rounds are sometimes called "under-bore capacity" (although they would not be with some such powder as Bullseye!). On the other hand, a case with so much capacity that the delta point is easily attainable even with the slowest propellants available today may be classed as "overbore". In the most common usage, a cartridge with a very large powder space behind a relatively small-diameter bullet is overbore, and if the bullets normally used in this case happen to be quite long, with high sectional densities, as in the 6.5mm magnums, the "overbore" condition is aggravated.

Virtually all the smallbore magnums (from 6mm to .30) can be thought of as over-bore capacity to some degree with the current crop of handloaders' powders. However, many non-belted cases, such as the .17 Rem., .243 wildcats on '06-sized cases, .25-06, and others are right on the line, so "overbore" is not a disease solely of magnums. Theoretically, the .50 Browning machine gun cartridge necked down to, say .35 or .40 caliber, might also be over-bore capacity with reloading powders.

If any ballistics engineer has written an intelligible essay on the physical reasons for the overbore phenomenon, I have not read it. There have been many rather off-hand "explanations" about the gases from a large powder charge not being able to get through a narrow bore fast enough, or some other such cause, but most of them can be applied as readily to a simple overload in any kind of cartridge. I make no effort to explain the reasons for "over" or "underbore", but only to make their effects understandable.
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Author:Wootters, John
Publication:Guns & Ammo
Article Type:column
Date:Jan 1, 1985
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